The present invention relates to methods of producing large, single crystals of silicon carbide with high crystalline quality suitable for use in semiconductor devices.
Silicon carbide (SiC) has a wide band gap, high stability and high thermal operating range that makes it a suitable material as a semiconductor for fabricating light sources, photodiodes, power diodes, field-effect transistors (FETs) and other semiconductor devices. In order to manufacture these semiconductor devices, the SiC is provided as large single crystals which are used to make SiC wafers. The quality of the semiconductor is highly dependent on the purity and structural characteristics of the silicon carbide single crystals.
One known process is commonly referred to as physical vapor transport (PVT) or the modified Lely method. This includes placing a SiC source material separated at a controlled distance from an SiC seed within a graphite crucible containing an inert gas such as argon. The gas is initially kept at a high pressure until the growth temperatures are obtained, and then the pressure is lowered to permit sublimation, mass transport of the source material in the form of vapor species (SiC2, Si2C, SiC and Si molecules), and then condensation and nucleation on the seed crystal. The source material and seed crystal are maintained at different temperatures (seed being at a lower temperature than source) to cause the mass transport of the vapor species from its original location to the seed crystal for condensation. When a grown single crystal is large enough or after a predetermined set time period, the temperatures are lowered and the gas pressure is raised to stop the growth. While some prior art methods attempt to provide constant temperatures, the crystal quality would decrease in the later stages of crystal growth when the source material approaches depletion, causing changes in temperature within the crucible and in turn affecting the structural integrity of the growing single crystal.
U.S. Pat. No. 4,866,005 to Davis et al., which corresponds to U.S. Pat. No. RE 34,861, European Patent Application Publication No. EPO 712150A 1 and European Application Publication No. 1143493A 1, discloses that controlling the polytype of the SiC source material, particularly SiC source powder, and controlling the flux of the source material improves the crystal quality even at the later stages of growth. The constant flux is also stated as constant flow of vaporized Si, Si2C and SiC2 per unit area per unit time. The flux of the source material to the seed can be controlled initially by maintaining constant temperatures, and when high temperature source material is used up or decreases, by changing the thermal gradient between source and seed (xc2x0 C./cm) by increasing the temperature of the source material or decreasing the distance between the source and seed. This provides for growth even after a high temperature portion of the source material runs out. Davis also teaches that pressure should be maintained at 10 Torr throughout growth. This process, however, is very cumbersome because it requires the monitoring of the flux of the source material to ensure constant flux.
U.S. Pat. No. 5,441,011 to Takahaski et al. discloses an improvement over the ""005 patent in that it teaches production of high quality crystal, also without a mix of polytypes, and that will grow at slower growth rates. To achieve this, however, initially the growth rate must be very high to avoid or outgrow black linear defects. Thus, it is disclosed that the source powder temperature should start and be maintained at an extremely high temperature and then be decreased continually throughout growth, which reduces the growth rate. The temperature of the seed crystal is fixed or decreases gradually so that the thermal gradient decreases gradually. The ""011 patent teaches that pressure is reduced from 600 Torr to 2 to 50 Torr (preferably 10 to 20 Torr) to instigate growth and the final pressure is maintained throughout growth to obtain a growth rate of in 0.2 to 2.5 mm/hr (preferably 0.4 to 1.6 mm/hr).
U.S. Pat. No. 5,968,261 is mainly directed to an improved crucible configuration that maintains nucleation on the seed crystal while eliminating uncontrolled nucleation on the carbon crucible surfaces during growth. This process places the seed on a stepped surface and/or uses an insulation pad to prevent source material contact with the crucible surfaces. The ""261 patent also discloses that the argon gas should be filled to a pressure over 100 Torr but less than atmospheric pressure because nucleation is initiated at 100 Torr. Then the temperatures are raised to 2100 to 2400xc2x0 C. at a gradient 10xc2x0 to 60xc2x0 C./cm and are held constant. Next, the pressure is decreased to increase the growth rate, and is then held at a final pressure between 0.1 to 50 Torr. While the ""261 patent discloses a way to eliminate unintentional nucleation and growth, it does not disclose how to increase the quality of the crystal by eliminating more of the defects in the SiC crystal.
In an article entitled xe2x80x9cNear-equilibrium growth of micropipe-free 6H-SiC single crystals by physical vapor transportxe2x80x9d by Schultze et al., a very specific four step process for growing SiC large single crystals is taught that claims to eliminate micropipes. Step 1 includes providing both the seed and source temperature at 2150xc2x0 C. (at thermal equilibrium) in a Lely furnace so that no thermal gradient exists while maintaining a very high argon pressure (well over 820 mbar) that prevents sublimation of the source material. While no transport of source material occurs at this step, it is claimed that lateral transport on the seed crystal itself exists and surface defects, e.g. polishing scratches and other visible surface defects, are annealed out. Step 2 includes lowering the pressure to 30 mbar (about 23 Torr) to provide a very low growth rate (about 0.23 mm/hr). Even though no thermal gradient was provided, growth occurred. The disclosure assumes that the partial pressure of silicon above the source material that was initially higher than the partial pressure at the seed caused the growth. However, this is highly speculative. Step 3 includes raising the temperature of the source material to 2180xc2x0 C. to provide a thermal gradient of 5 K/cm to increase the growth rate to an acceptable level while maintaining the pressure at 30 mbar. This step yielded growth at about 0.09 mm/hr. Finally, the preferred Step 4 included maintaining the same temperatures but lowering the pressure to 5 mbar to raise the growth rate. Again, it is claimed that no precipitates, defects or micropipes are formed and a growth rate of 0.27 mm/hr was achieved.
While the Schulze article asserts that it can eliminate micropipes, the change in temperature during the growth step causes other structural defects in the crystal. Specifically, any change in temperature during growth causes a change in polytype structures (such as 3C or 6H). A semiconductor with multiple polytypes causes variations or inconsistencies in crystal characteristics and quality.
In order to provide a crystal of consistent quality and characteristics in a process that does not require the monitoring, generating and maintaining of a substantially constant flow of vaporized Si, Si2C, and SiC2 per unit area per unit time from the source, and in fact intentionally varies the flow of the vaporized SiC, in one aspect of the present invention, a constant temperature process is used that carefully controls SiC single crystal growth by varying the pressure to vary the growth rate rather than using the temperature to vary the growth rate. Growth rate is measured as mass or volume increase per unit time. Such a system provides excellent control of the growth rate of the crystal while providing a relatively non-varying supply of SiC vapor species. Specifically, a method of growing a silicon carbide single crystal on a silicon carbide seed crystal in an inert gas environment includes the step of establishing the seed crystal temperature at a growth temperature Tseed and establishing the temperature of source material at a growth temperature Tsource that is higher than Tseed to define a thermal gradient therebetween. The process also requires maintaining constant seed temperature and constant source temperature throughout substantially the entire growth period of the single crystal. The growth period begins when the seed crystal and source material reach Tseed and Tsource, respectively. Another step requires changing the pressure of the inert gas during the growth period to control the growth rate of the crystal without changing any temperatures once growth of the single crystal has started.
In another aspect of the present invention, it has been determined that the use of an initial low growth rate prevents introduction of growth defects at the seed/crystal interface and grows a base for the single crystal with a very low amount of defects. After the crystal base is established, a higher growth rate is provided to grow the remainder of the single crystal, which results in a very high quality SiC single crystal. The different growth rates are achieved by providing an initially higher gas pressure for base growth before lowering the gas pressure to increase the growth ratexe2x80x94all under essentially constant temperature. More particularly, as part of the step of decreasing the pressure, the present invention also includes a step or substep reducing the pressure to a first pressure P0, where transport of SiC source material to the seed still occurs at very low rates, and holding the pressure at P0 for a duration which is adequate to grow a low defect base for the crystal on the seed. After growing the base, the pressure is reduced again to a second pressure P1 to continue growing the remainder of the single crystal.